Make gitlab:update_project_templates rake task more robust and improve tests

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Summary

Spun off from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31592#note_203414463

The rake task gitlab:update_project_templates could be more robust, and the tests could be better

Improvements

• easy: the task has grown fairly large and could be extracted into a class
• somewhat difficult: the task could be refactored to not require a database or a full rails environment
• easy: the tests could cover not just that export "works" but also that projects can be created from the files it generates
• easy: the "unit" tests should use fixtures instead of downloading actual source code archives From GitLab.com
• difficult: the whole project template feature could be simplified, leading to a simpler vendoring process, see https://gitlab.com/gitlab-org/gitlab-ce/issues/66068

Risks

Project templates can break in various ways that might not be covered by tests.

Involved components

Optional: Intended side effects

Optional: Missing test coverage